Enhanced Fenton-like process over Z-scheme MoO3 surface decorated with Fe2O3 under visible light

Photocatalysts consisting of Z-scheme heterojunctions are commonly used in wastewater treatment due to their exceptional reactivity in photocatalysis and highly efficient visible-light utilization. In this work, Fe2O3-decorated MoO3 rods were synthesized through a two-step method and their photodegradation of methylene blue (MB) was evaluated. The Fe2O3/MoO3 rods were characterized by XRD, SEM, micro-Raman, XPS, UV–Vis DRS, and PL to investigate their structural, morphological, and optical properties. The results indicate that the photodegradation efficiency of Fe2O3/MoO3 improved through a reduction in the gap energy and persistence of a 1D hexagonal prism structure. The degradation rate of MB was enhanced from 31.7 to 91.5% after irradiation for 180 min owing to electron–hole separation and Fenton-like process. Formation of the OH radical is a key factor in the photodegradation reaction and with the addition of H2O2 the efficiency can further improve via a Fenton-like mechanism. Furthermore, the Z-scheme mechanism concurrently delineated. The Fe2O3/MoO3 rod composites were also found to retain high photocatalytic efficiency after being reused five times, which may be useful for future applications.

Chithambararaj et al. 13 prepared h-MoO 3 and α-MoO 3 nanocrystals and found that the degradation process was slower for α-MoO 3 .This was attributed to a reduction in electron-hole pairs in the electronic band structure and also the enhancement of h-MoO 3 due to its 1D hexagonal prism nanostructure.Li et al. 14 prepared h-MoO 3 from an α-MoO 3 precursor using NaNO 3 and transformed h-MoO 3 back to α-MoO 3 nanobelts using a hydrothermal technique.They also found that h-MoO 3 nanorods had greater catalytic activity for photodegrading methylene blue (MB) due to its reduced band gap and lower electron-hole recombination rate.Based on the literature, it is apparent that the 1D hexagonal prism structures in MoO 3 are very important for enhancing the photodegradation ability 15 .
Many types of p-n heterojunction composite metal oxides were designed to adjust their band gap energy and hence improve applicability in solar cells, gas sensing, and photocatalysis.Charge transfer reduces the recombination rate of photogenerated charge carriers, greatly improving their reactivity.The Z-scheme mechanism can also separate charge carriers to keep a greater negative potential of electrons (e − ) and a greater positive potential of holes (h + ), thus providing more energy for photocatalytic reactions [17][18][19] .The semiconductor Fe 2 O 3 is commonly used for photodegradations because of its narrow band gap and suitable band position.It is also used to synthesize composite materials with other metal oxide semiconductors in photocatalytic applications.For example, Fe 2 O 3 /TiO 2 photoanodes have an increase of 40% in solar conversion efficiency with fast electron transport and a short electron lifetime 20 .Toluene sensing is greatly improved by a heterojunction between NiO and α-Fe 2 O 3 composed of flower-like hollow structured composites manufactured by a hydrothermal process 21 .P-type MoO 3 nanostructures were incorporated into n-type TiO 2 nanofibers via an electrospinning process 22 , which resulted in higher photocatalytic activities due to the heterojunction effect and band gap alignment.α-MoO 3 nanorods loaded with α-Fe 2 O 3 nanoparticles on the surface were synthesized via a two-step hydrothermal method by Xiao et al. and used in the photodegradation of tetracycline.This led to enhanced performance because the heterojunction between MoO 3 and Fe 2 O 3 allowed efficient electron-hole transport with the main participation of hydroxyl radical and hole 23 .Core-shell α-MoO 3 /α-Fe 2 O 3 nanostructure composites were fabricated by a hydrothermal method and were shown to be a promising xylene sensor with a three times higher response than α-MoO 3 at 206 °C24 due to quick adsorption and desorption at 1D nanostructures and the lowered effective barrier height of the heterojunction.Fe 2 O 3 •nMoO 3 nanowires with urchin-like and bowknot-like nanostructures, exhibiting 97.8% efficiency for Congo red degradation, were also synthesized 25 .Ternary Fe 2 O 3 /MoO 3 /AgBr nanoparticles were synthesized and uncovered the important role of superoxide radicals and holes in the decomposition of AB92 dye, giving a pseudo-first-order rate constant of 18.23 × 10 −2 min −1 for the degradation reaction 26 .
Based on the importance of 1D structures and the enhancement of photodegradation with heterojunction structures, in this study we developed a two-step microwave-assisted synthetic method to synthesize 1D h-MoO 3 hexagonal prisms decorated with Fe 2 O 3 nanoparticles.This simple and rapid method for preparing these materials is an energy saving and environmentally friendly approach.Furthermore, the highly efficient photocatalysts, prepared via a microwave-assisted synthetic method, have the potential to be reused, while retaining stability.The synthesized materials were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), Raman, and UV-Vis diffuse reflectance spectroscopy (DRS).Finally, photodegradation of MB was evaluated for the synthesized composite Fe 2 O 3 /MoO 3 structure and the photodegradation mechanism was investigated using various types of sacrificial agents.

Material preparation
The composite Fe 2 O 3 /MoO 3 material was synthesized by a two-step procedure beginning with the synthesis of metastable h-MoO 3 hexagonal prisms using a microwave heating method, followed by a mixing calcination process with different amounts of Fe 2 O 3 decorated onto the MoO 3 .

Synthesis of h-MoO 3 by microwave heating
An aqueous solution of 0.04 M AHM was prepared in deionized water and stirred until all AHM had dissolved.Subsequently, 10 mL of 2.2 M HNO 3 (aqueous solution) was added to the AHM solution and heated in a Milestone STARTS microwave digestion system at 70 °C for 30 min.After centrifugation at 20,000 RPM, the white powder was washed with deionized water and ethanol several times and dried in an oven at 60 °C overnight.The white hexagonal prism of h-MoO 3 was obtained and is referred to as h-MO in the rest of the paper.

Synthesis of Fe 2 O 3 /MoO 3 by mixing calcination
0.176 g of Fe(NO) 3 •9H 2 O was added to 50 mL of deionized water and stirred until dissolved.This solution is denoted as solution A. Four different volumes (1, 4, 7, and 14 mL) of solution A were then diluted with deionized water to a final volume of 100 mL.0.15 g of h-MoO 3 was added to each solution and stirred at 40 °C for 90 min.An orange powder was obtained after centrifuging at 20,000 RPM and was washed with deionized water and ethanol several times and then dried in an oven at 60 °C overnight.Finally, the dried sample was calcinated at

Photocatalytic degradation
The prepared samples were investigated for photocatalytic degradation of MB in aqueous solution at room temperature using a PR-2000 photochemical reactor under visible-light irradiation.The following experimental procedure was used: 20 mg of catalyst was suspended in 100 mL of 8 ppm MB aqueous solution.0.1 mL of 35 wt% H 2 O 2 was added and the sample was stirred in the dark for 60 min to reach sorption-desorption equilibrium.The photocatalytic reaction was then performed by irradiating the mixture with 420 nm light using a 128 W fluorescent tube light.During irradiation, 3 mL of solution was removed at designated time intervals and centrifuged at 20,000 rpm for 5 min.The supernatant solutions were analyzed by a Hitachi U-3010 UV-Vis spectrophotometer, and the spectra were recorded with the characteristic absorption peak of MB at λ = 664 nm.

XRD studies
The crystal structures of the different samples were characterized by XRD and are shown in  www.nature.com/scientificreports/This could possibly be due to the low amount of decorated Fe 2 O 3 in the sample or poor crystallinity of Fe 2 O 3 synthesized through this method 27 .

SEM analyses
The SEM images of h-MO (a), h-MO-0 (b), and various Fe 2 O 3 /MoO 3 composites (c to f), are shown in Fig. 2. In Fig. 2a, h-MO takes the form of 1D hexagonal prism rods.The individual rods are polygon-like crystals with hexagonal cross sections and well-developed facets.Impurity particles are not apparent, and the average diameter is about 500-800 nm, with the length approximately between 5 and 10 µm.From Fig. 2b, the h-MoO 3 sample calcinated at 400 °C for 2 h in air (h-MO-0) was found to transform to the orthorhombic α-MoO 3 crystal phase, and sheet-like structures are observed instead of hexagonal prism nanorods.As the amount of Fe 2 O 3 decoration increases (Fig. 2c-f, from h-MO-1 to h-MO-14), the morphology of the Fe 2 O 3 /MoO 3 composites evolved from sheet-or plate-like to hexagonal prism rods.This indicated that despite the increased amount of Fe 2 O 3 (i.e., h-MO-7 and h-MO-14) decorated on the surface of the original h-MO hexagonal prism rod, the rod-like morphology is retained even as the crystal phase of MoO 3 transformed from h-MoO 3 to α-MoO 3, as observed from the XRD data.The phase transformation from h-MoO 3 to α-MoO 3 can be explained by a dissolution-recrystallization mechanism 28 , where the metastable h-MoO 3 rod dissolved partially, breaking the zigzag chains of the cis-interlinked MoO 6 octahedron, and recrystallizing to form the α-MoO 3 sheet that is constructed by the corner-sharing distorted MoO 6 octahedron with van der Waals forces.However, with Fe 2 O 3 , the hexagonal prism shapes are preserved while phase transformation takes place, differing from a previous study 14 where the h-MoO 3 rod dissolved and recrystallized, forming α-MoO 3 nanobelts at 400 °C.As the concentration of Fe 2 O 3 increased, we observed some nanoparticles sitting on the surface of the MoO 3 hexagonal prisms.

TEM analyses
The morphology of h-MO-0 (a) and h-MO-7 (b) was investigated by TEM and shown in Fig. 3, respectively.All are rod-shaped, but they differ in size.When h-MO is calcinated without Fe 2 O 3 added to form h-MO-0, the size changes from microrods of approximately 1 μm in length and 200 nm in width (Fig. S1) to nanorods ranging from 500 to 1000 nm in length and 100-200 nm in width.However, the rods become wider with an increasing

Raman analyses
The Raman spectra of all products are shown in Fig. 4. For h-MO, the observed peaks at 118, 131, 176, 218, and 248 cm −1 are all indexed to h-MoO 3 29 , which corresponds to the skeletal modes of tetrahedral MoO 4 .The peaks at 313, 395, 413, 489, and 689 cm −1 correspond to O-Mo-O vibrations, and the peaks at 882, 900, 911, and 974 cm −1 correspond to the Mo=O bond.The Raman results agreed with the previous XRD and SEM findings that the h-MO sample is in the h-MoO 3 hexagonal prism form.For the h-MO-0, h-MO-1, h-MO-4, h-Mo-7, and h-MO-14 samples, the peaks located at 81, 94, 113, 124, 152, and 213 cm −1 are assigned to α-MoO 3 , which corresponds to MoO 4 translational and rotational chain modes.The 195, 241, and 286 cm −1 peaks correspond to O=Mo=O twisting and wagging modes.Peaks at 336, 375, 468, and 663 cm −1 correspond to O-Mo-O bending, scissoring, stretching and bending, and stretching modes, respectively.Finally, the sharp and intense peaks at 818 and 994 cm −1 correspond to Mo=O stretching modes 30 .The Raman spectra results indicate that the MoO 3 orthorhombic phase formed after the calcination process, even though the heating temperature of 400 °C was below the phase transformation temperature of 419 °C31 or 430 °C32 for metastable h-MoO 3 to stable α-MoO 3 .www.nature.com/scientificreports/

XPS analyses
XPS spectra were used to analyze the chemical components and valence states of the prepared samples.As shown in Fig. 5a, the elements Mo, O, and C are present in the h-MO-0 sample.However, in addition to the above elements, there is also presence of Fe in h-MO-7 and h-MO-14 samples.Figure 5b reveals the two peaks of Mo 3d located at around 236.1 and 232.9 eV, which correspond to Mo 3d 3/2 and Mo 3d 5/2 , respectively, suggesting the presence of Mo in the 6 + oxidation state.For the O 1s spectra shown in Fig. 5c, the peak observed at around 530.8 eV can be assigned to oxygen present in the lattice.From the results above, we can confirm that the samples all contain MoO 3 .In Fig. 5d, the Fe 2p spectra consist of two peaks at around 725.3 and 711.6 eV corresponding to Fe 2p 1/2 and Fe 2p 3/2 , respectively.Furthermore, a satellite peak at 719.9 eV characteristic of Fe 3+ is revealed; this indicates that the h-MO-7 and h-MO-14 samples also contain Fe 2 O 3 .Moreover, the chemical shift in Fig. 5b,c can be attributed to the interaction between MoO 3 and Fe 2 O 3 , where new bonds were formed to form the heterojunction, which enhances photocatalytic efficiency 33 .

Optical properties
The photoabsorption properties of the samples were characterized by UV-Vis DRS.From Fig. 6a, the absorption edge of h-MO is around 440 nm, and the absorption maximum is around 310 nm.For all h-MO-x samples, we observed red-shifted spectra compared with the h-MO sample.We also found that both the position of the absorption peak and absorption onset are red-shifted as the amount of Fe 2 O 3 increased.However, h-MO-14 gave a similar result to h-MO-4, which was an exception.The band gap energy can be determined on the basis of the Tauc plot method [34][35][36] : where α, h, ν, E g , and A n are the absorption coefficient, Planck constant, light frequency, optical energy gap, and the probability parameter for the transition, respectively.In addition, n is determined by the type of optical transition in a semiconductor, where n = 1/2 for allowed direct transitions and n = 2 for indirect transitions 37 .
For MoO 3 , a direct transition gap is observed, and so the value of n is set to 2.
The gap energy can be given from the linear region of the plot (αhν) 2 versus hν.From Fig. 6b, the band gap energies of h-MO, h-MO-0, h-MO-1, h-MO-4, h-MO-7, and h-MO-14 were estimated to be 3.12, 3.06, 3.08, (1) PL was used to measure the recombination rate of photogenerated charge carriers in the metal oxide semiconductors.In Fig. 6c, h-MO-0 displayed the strongest emission intensity.As the Fe 2 O 3 content increased, the emission intensities are lower than h-MO-0 (pure MoO 3 ).h-MO-0 and h-MO-7 demonstrated the lowest peak, indicating that the separation efficiency of photogenerated charge carriers in the composites significantly increased due to the formation of the heterojunction, resulting in improved photocatalytic performance.However, the emission intensity of h-MO-14 was higher than that of h-MO-7 since an excess of Fe 2 O 3 covering the active sites of MoO 3 prevented MoO 3 from forming photogenerated carriers.Therefore, the separation efficiency is decreased compared to h-MO-7 19,38,39 .
The charge transfer ability of the synthesized samples was measured using EIS, as shown in Fig. 6d.The arc radius of h-MO-7 was the smallest among the samples tested.A smaller arc radius indicates a more efficient charge transfer, suggesting that h-MO-7 facilitates faster electron transfer across the heterojunction compared to the other samples.This result is consistent with the PL results, indicating that when the material possesses better charge transfer between heterojunctions there is an increase in the electron-hole separation efficiency.This characteristic is beneficial for enhancing photocatalytic efficiency [40][41][42][43] .

Evaluating the photocatalytic degradation of MB
The photocatalytic performance of the synthesized samples was investigated with 420 nm visible light, as shown in Fig. 7a.As a baseline, a dark adsorption experiment was performed by stirring for 60 min to achieve a preequilibrium state before visible-light irradiation.After 180 min of irradiation, we found that pure α-MoO 3 (h-MO-0) only degraded 31.7% of MB, while the Fe 2 O 3 /MoO 3 composites, h-MO-1, h-MO-4, h-MO-7, and h-MO-14, degraded 42.1, 87.3, 91.5, and 81.4% of MB, respectively (Fig. 7b).The degree of MB degradation increased as the amount of Fe 2 O 3 increased, except for the case of h-MO-14.h-MO-14 had higher Fe 2 O 3 content but lower efficiency than h-MO-7 probably because Fe 2 O 3 nanoparticles are distributed and coated on the surface of the MoO 3 rods, where electrons recombine with holes on the surface of Fe 2 O 3 , making reactions with hydrogen peroxide difficult 22,44 .h-MO-7 has the greatest degradation efficiency of 91.5% which is about three times larger than that of undecorated MoO 3 (h-MO-0), which could be explained by the lower gap energy, as indicated by the UV-Vis analysis.To further investigate the photocatalytic activity of the prepared samples, the Langmuir-Hinshelwood kinetics model was applied to evaluate the degradation rate and expressed in the following equation 45-47 : where C o is the initial concentration of MB, C is the concentration of MB at time t (in minutes), and k is the unimolecular photodegradation rate constant.Photodegradation rate constants were calculated by plotting ln(C 0 /C) versus irradiation time and given in Fig. 7c.We found that the rate constant of h-MO-7 (0.01288 min −1 ) is about seven times that of the undecorated h-MO-0 sample (0.00183 min −1 ).Compared with other research (Table S1), these catalysts show good performance 14,23,31,48,49 .

Fenton-like processes
From Fig. 8, it can be seen that addition of 5 mL of hydrogen peroxide to the h-MO-14 (denoted as h-MO-14-H) and h-MO-7 (denoted as h-MO-7-H) samples further enhanced the photodegradation process.Without additional H 2 O 2 , MB is fully degraded after 150 min with h-MO-7 and h-MO-14.However, for the h-MO-7-H and h-MO-14-H samples, MB is fully degraded in about 50 min.This might be due to both the Fenton reaction, and the Fenton-like mechanism taking place: where more hydroxyl radicals are produced to fragmentize the MB [50][51][52] due to the higher content of Fe 2 O 3 in h-MO-14-H.
(3) ).Sacrificial agents were added to identify factors in the degradation reaction.In our kinetic experiments, H 2 O 2 served as an electron sacrificial agent that consumes electrons, and so no further investigation was conducted on the effect of electrons.IPA served as the sacrificial agent for hydroxyl radicals, while EDTA and BQ were the sacrificial agents for holes and superoxide ions, respectively 53,54 .In Fig. 9, when adding IPA the h-MO-7 sample containing 0.1 mL H 2 O 2 , the efficiency decreased significantly due to the consumption of hydroxyl radicals.Adding EDTA, which consumes holes, also decreased the efficiency of photodegradation.The efficiency decreased only by a small amount with the addition of BQ.From these tests, we conclude that production of •OH radicals is the primary factor in the photodegradation mechanism and that hole generation is a secondary factor.

Photodegradation mechanism
The enhanced photocatalytic activity of the Fe 2 O 3 /MoO 3 composites can be explained by the relative band positions of the Fe 2 O 3 /MoO 3 composites.The band energies of two metal oxide semiconductors can be estimated by the following expressions 55 : where E VB is the valence band edge potential, Χ is the geometric mean of the constituent atoms' electronegativities, E 0 is the energy of the free electrons on the hydrogen scale (~ 4.50 eV), and E g is the band gap of the semiconductor.On the basis of previous reports, E VB and E CB of Fe 2 O 3 were calculated to be 2.49 and 0.29 eV, respectively, and the E VB and E CB values of MoO 3 were found to be 3.33 and 0.47 eV 33,53 , respectively.As shown in (4)  Fig. 10a,b, type II and Z-scheme modes are two possible synergistic modes in this photocatalytic process.From Fig. 10a, it can be deduced that e − on the CB of Fe 2 O 3 would flow to the CB of MoO 3 , and h + on the VB of MoO 3 would transfer to the VB of Fe 2 O 3 .However, the more negative potential of •OH/H 2 O 2 (0.38 eV vs. NHE) than the E CB of MoO 3 and the more positive potential of •OH/H 2 O (2.73 eV vs. NHE) than the E VB of Fe 2 O 3 make formation of hydroxyl radicals difficult.Moreover, the removal of e − on the CB of Fe 2 O 3 also interferes with the Fenton-like reaction, contradicting our earlier results that indicate that the photodegradation reaction involves a Fenton-like mechanism when H 2 O 2 is added.Thus, a type-II-mode-based combination between MoO 3 and Fe 2 O 3 is excluded.In a Z-scheme mode, e − on the CB of MoO 3 can combine with h + on the VB of Fe 2 O 3 directly and prevent the remaining e − and h + from recombining.Because the E CB of Fe 2 O 3 is more negative than the potential of •OH/H 2 O 2 (0.38 eV vs. NHE) and the E VB is more positive than the potential of •OH/H 2 O (2.73 eV vs. NHE), the photodegradation reactions can be carried out.Furthermore, preservation of e − on the CB of Fe 2 O 3 improves the probability of the Fenton-like mechanism, consistent with previous results.Therefore, we conclude that the Z-scheme mode is the mechanism that takes place [56][57][58][59] .

Catalytic recycle
The stability of the h-MO-7 catalyst was investigated by a recycle test.We added 30 mg of sample into 100 mL of 8 ppm MB aqueous solution with 0.1 mL of 35 wt % hydrogen peroxide.The mixture was stirred in the dark for 60 min to achieve adsorption-desorption equilibrium.The mixture was irradiated with 128 W, 420 nm fluorescent light for 180 min.The sample was dried in an oven and reused for the same reaction five times.As shown in Fig. 11, we found that the efficiency of photodegradation was about 98% after the fifth cycle, which could be due to sample loss during the recycling process.Thus, the stability and potential for reusing the h-MO-7 and Fe 2 O 3 /MoO 3 composite catalysts are promising.

Conclusion
We have successfully synthesized an Fe 2 O 3 /MoO 3 composite semiconductor photocatalysts using a two-step method that consists of microwave synthesis and a mixing calcination process.With a sufficient amount of Fe 2 O 3 , the shape of the Fe 2 O 3 -decorated hexagonal rod prism structure was preserved even though the crystal phase transformed from hexagonal h-MoO 3 to orthorhombic α-MoO 3 .The synthesized composites were applied in the photodegradation of MB, and the best efficiency of 91.5% was found to belong to h-MO-7, compared to 31.7%

Figure 4 .
Figure 4. Raman spectra of the different samples.

Figure 8 .
Figure 8. Photocatalytic degradation of methylene blue with addition of H 2 O 2 under 420 nm irradiation.

Figure 10 .
Figure 10.(a) Type-II and (b) Z-Scheme diagram of proposed photocatalytic mechanism and energy band gap of MoO 3 and Fe 2 O 3 .